In the manufacture of tissue products, such as facial tissue, bath tissue, paper towels, dinner napkins and the like, a wide variety of product properties are imparted to the final product through the use of chemical additives applied in the wet end of the tissue making process. Two of the most important attributes imparted to tissue sheets and tissue products through the use of wet end chemical additives are strength and softness. Specifically for bulk softness, a chemical debonding agent is normally used. Such debonding agents are typically quaternary ammonium compounds containing long chain alkyl groups. The cationic quaternary ammonium entity allows for the debonding agent to be retained on the cellulose via ionic bonding to anionic groups on the cellulose fibers. The long chain alkyl groups provide softness to the tissue sheet by disrupting fiber-to-fiber hydrogen bonds in the tissue sheet.
The use of such debonding agents is broadly taught in the art. Such disruption of fiber-to-fiber bonds provides a two-fold purpose in increasing the softness of the tissue sheet. First, the reduction in hydrogen bonding produces a reduction in tensile strength thereby reducing the stiffness of the sheet. Secondly, the debonded fibers provide a surface nap to the tissue sheet enhancing the “fuzziness” of the tissue sheet. This tissue sheet fuzziness may also be created through use of creping as well, where sufficient interfiber bonds are broken at the outer surface of the tissue sheet to provide a plethora of free fiber ends on the surface of the tissue sheet. Both debonding and creping increase levels of lint and slough in the tissue product. Indeed, while softness increases, it is at the expense of an increase in lint and slough in the tissue sheet relative to an untreated control. It can also be shown that in a blended (non-layered) tissue sheet that the level of lint and slough is inversely proportional to the tensile strength of the tissue sheet. Lint and slough can generally be defined as the tendency of the fibers in the tissue sheet to be rubbed from the tissue sheet when handled.
It is also known in the art to use a multi-layered tissue structure to enhance the softness of the tissue sheet. In such a tissue structure, a thin layer of strong softwood fibers is used in the center layer to provide the necessary tensile strength for the tissue product. The outer layers of such a tissue structure are composed of the shorter hardwood fibers, which may or may not contain a chemical debonder. A disadvantage to using such layered structures is that while softness of the tissue sheet is increased, the mechanism for such increase is believed due to an increase in the surface nap of the debonded, shorter fibers. As a consequence, such tissue structures, while showing enhanced softness, do so with a trade-off of an increase in the level of lint and slough.
It is also known in the art to concurrently add a chemical strength agent in the wet-end to counteract the negative effects of the debonding agents. In a blended tissue sheet, the addition of such agents reduces lint and slough levels. However, such a reduction occurs at the expense of surface feel and overall softness and becomes primarily a function of sheet tensile strength. In a layered tissue structure, strength chemicals are typically added to the center layer. While this perhaps helps to give a tissue sheet with an improved surface feel at a given tensile strength, such tissue structures actually exhibit higher slough and lint at a given tensile strength, with the level of debonder in the outer layer being directly proportional to the increase, in lint and slough. U.S. Pat. No. 6,488,812 issued on Dec. 3, 2002 to Shannon et al. discloses low slough tissue products made with synthetic polymers (acrylamides containing hydrophobic moieties). These synthetic polymers, while reducing the amount of slough compared to traditional debonders, may still show an increase in slough with decreasing tensile strength.
Another well known process for improving softness in tissue is to topically apply a softener to the surface of the tissue sheet. A broad range of softeners is known in the art. An especially effectively topical softener is polysiloxane. For example, polysiloxane treated tissue sheets are described in U.S. Pat. No. 4,950,545, issued on Aug. 21, 1990 to Walter et al.; U.S. Pat. No. 5,227,242, issued on Jul. 13, 1993 to Walter et al.; U.S. Pat. No. 5,558,873, issued on Sep. 24, 1996 to Funk et al.; U.S. Pat. No. 6,054,020, issued on Apr. 25, 2000 to Goulet et al.; U.S. Pat. No. 6,231,719, issued on May 15, 2001 to Garvey et al.; and, U.S. Pat. No. 6,432,270, issued on Aug. 13, 2002 to Liu et al., which are incorporated by reference to the extent that they are non-contradictory herewith. A variety of substituted and non-substituted polysiloxanes may be used.
While polysiloxanes are exceptionally good at improving softness there are some drawbacks to their use. Polysiloxanes are relatively expensive. Only polysiloxane on the outermost surface of the tissue sheet is believed to contribute to tactile softness. Polysiloxane present in the z-direction contributes only to bulk softness, i.e., its ability to impact softness is dependent only on its ability to reduce interfiber hydrogen bonding. Interfiber hydrogen bonding may be more efficiently controlled with traditional quaternary ammonium debonding agents.
Polysiloxanes are also effective debonders. Typically polysiloxanes are applied to the tissue sheet as an emulsion in a printing or spraying process. Some polysiloxane penetrates the z-direction of the tissue sheet and reduces the relative bonded area of the tissue sheet, especially at the topmost surface of the tissue sheet. This reduction in relative bonded area is typically accompanied by a loss of tensile strength. Along with this loss in strength comes an increase in the amount of lint and slough in the tissue sheet or tissue product.
It is known to add certain binders to the tissue sheet to mitigate the tensile loss and higher lint associated with application of the polysiloxane. For example see U.S. Pat. No. 5,389,204, issued on Feb. 14, 1995 to Ampulski; U.S. Pat. No. 5,385,643, issued on Jan. 13, 1995 to Ampulski; and, U.S. Pat. No. 5,573,637, issued on Nov. 12, 1996 to Ampulski, the disclosures of which are herein incorporated by reference to the extent that they are non-contradictory herewith. Such binders are typically limited to traditional paper strength agents known in the art and specific examples typically still more limited to traditional dry strength starches. By combining traditional paper strength agents with polysiloxanes, the traditional paper strength agents may mitigate some of the tensile loss associated with the debonding action of the polysiloxane. However such a combination introduces a large number of hydrogen or covalent bonds to the tissue sheet, thereby increasing the stiffness of the tissue sheet, reducing the softness of the tissue sheet. Furthermore, these materials themselves have high glass transition temperatures that further increase the stiffness of the sheet. Furthermore, aldehyde, epoxy, and azetidinium functional strength agents may react prematurely with functional groups, if present, on the polysiloxane, particularly amino-functional polysiloxanes, to form crosslinked gels that may hinder application to the tissue sheet. Furthermore, other traditional paper strength chemicals are not compatible with polydimethylsiloxanes typically used in tissue making processes. This lack of compatibility may cause separation into two distinct phases, a polysiloxane phase and a polymer phase. The phases are discontinuous on a macro scale, allowing the polysiloxane to migrate into the tissue sheet and hence further reduce the softness of the tissue sheet and any resulting tissue products.
Typically the binders must be added as a separate step either as a second post treatment or in the wet end of the tissue making process. Although concurrent addition of polysiloxane has been noted in the art, such practice is difficult as incompatibility of the binder and the polysiloxane make simultaneous application detrimental to the tissue making process.
Latex binders, specifically styrene-butadiene rubber latexes, may be used in conjunction with polysiloxane to mitigate strength loss. The application of latex binders combined directly with polysiloxanes has not been taught in the art. Commonly used tissue-making latex binders are not compatible with polysiloxanes and polysiloxane emulsions and therefore can not be added to the tissue sheet concurrent with the polysiloxane. Hence, again a two separate step process is required, one step for applying the polysiloxane and one step for applying the latex binder. Furthermore, these latex binders are in the form of emulsions containing a high level of surfactant. The latex binders are not of themselves water dispersible. The emulsions are susceptible to breaking, causing a film of the latex binder to develop on processing equipment. This film of latex binder continues to deposit on equipment to the point where shutdown and clean-up of the equipment is required. As the latex binders are not water dispersible, clean-up may be time consuming, costly and environmentally unfriendly.
Therefore, there is a need for a means of reducing lint and slough in soft tissue sheets containing polysiloxanes while maintaining softness and strength of the tissue sheets. There is furthermore a need for reducing lint and slough at the same time as applying the polysiloxane. There is a further need to mitigate the z-directional penetration of the polysiloxane when applied to tissue sheets so as to improve softness of the tissue sheet using lower levels of polysiloxane.
It is an objective of the present invention to design polysiloxane emulsions for treating tissue and towel products possessing an ability to reduce lint and slough while maintaining the softness of the treated tissue sheet. It is a further objective of the present invention to make soft, low lint and slough tissue products such as sanitary bath tissue, facial tissue, paper towels and the like via the application of such polysiloxane emulsions containing compatible binders. It is a further objective of the present invention to apply the binder simultaneously with the polysiloxane as part of a single formulation package. Still furthermore, it is an objective of the present invention to improve the tactile softness of tissue sheets by applying a polysiloxane to the surface of tissue sheet in a manner that minimizes the z-directional penetration of the polysiloxane in the tissue sheet. In this manner equivalent softness of the tissue sheet may be obtained at lower application levels of polysiloxane.